Word Count: 576 TA2 pure titanium possesses outstanding resistance to chloride ion corrosion in seawater and brine media, and is widely adopted for heating elements in desalination, marine engineering and salt chemical industry. Nevertheless, under the combined coupling of tensile residual stress, sustained high temperature and concentrated chloride solution, titanium material will still produce brittle stress corrosion cracking (SCC). Such failure has no obvious metal thinning on the exterior, and cracks expand rapidly along the crystal structure, easily triggering sudden tube penetration and medium leakage without early warning. This paper elaborates the formation mechanism and full-process prevention scheme.
1. Mechanism of Titanium Stress Corrosion Cracking
Titanium spontaneously forms compact TiO₂ passive film in neutral chloride medium to isolate matrix corrosion. When the pipe body bears tensile stress from processing or assembly, micro-tears appear on the surface oxide film. Chloride ions continuously invade the crack tip, dissolving fresh titanium substrate to form a local acidic microenvironment. Stress concentrates at the front end of tiny cracks and pushes cracks to extend rapidly along the grain boundary or transgranular direction. The whole fracture process belongs to brittle failure, with almost no plastic deformation, so it is difficult to discover hidden dangers through regular appearance inspection. Three indispensable conditions for SCC: tensile stress, medium temperature above 70℃, high concentration chloride medium. Missing any one condition can greatly reduce cracking probability.
2. Main Sources of Tensile Stress on Titanium Heating Tubes
(1) Cold Bending Residual Stress
U-shaped, L-shaped and coiled titanium heating tubes are cold bent and formed. The outer arc of the bending area generates permanent tensile stress, which is the most prone position for SCC. Too small bending radius will cause severe stress concentration and sharply shorten the safe service cycle.
(2) Welding Thermal Residual Stress
Argon arc welding produces uneven heating and cooling. Weld seam and heat-affected zone retain large shrinkage tensile stress. If the welding protection is incomplete, oxygen and nitrogen embrittlement will occur on the weld, further reducing the critical stress threshold for cracking.
(3) Assembly External Stress
Forced alignment during flange locking, rigid clamping and pipeline pulling apply additional tensile load to the pipe body, superimposed with inherent processing stress, making the material enter the sensitive SCC interval at a lower temperature.
(4) Thermal Cycle Alternating Stress
Frequent startup and shutdown lead to repeated expansion and contraction of the pipe wall, forming cyclic alternating stress, which accelerates the initiation and propagation of microcracks.
3. Hierarchical Prevention and Control Technical Measures
① Standardize Bending Process Parameters
Specify the minimum bending radius not less than 3 times the outer diameter of the titanium pipe to avoid excessive stretching deformation at the bend. After bending forming, carry out low-temperature stress relief annealing to eliminate forming residual stress from the source.
② Strict Double-Sided Argon Shielded Welding
Adopt full backside argon protection during welding to prevent weld embrittlement. For heating tubes used in saturated brine above 90℃, perform post-weld solid solution heat treatment to homogenize microstructure and eliminate welding residual stress.
③ Adopt Flexible Installation Structure
Prohibit forced assembly and rigid fixed constraints; use PTFE elastic gaskets and reserved expansion gaps during flange connection to avoid introducing extra external tensile stress.
④ Reasonably Control Power Density
Optimize surface power load to avoid local overheating of the pipe wall, keep the long-term operating temperature below the sensitive temperature range of titanium SCC as much as possible, and cut off the temperature inducement condition.
⑤ Optimize Equipment Shutdown Management
Completely drain the residual brine inside the tank during long-term shutdown and standby to prevent chloride ion enrichment under static medium and avoid forming a high-corrosion microenvironment.
4. Material Upgrade Direction for Ultra-Harsh Working Conditions
If the working condition is saturated brine with temperature stably higher than 120℃ and complex multi-source stress, TA1 industrial pure titanium with lower impurity content and better toughness can be selected to improve anti-SCC performance. It should be emphasized that titanium materials are completely unable to resist fluoride-containing media, and cannot be used in working conditions with hydrofluoric acid and fluoride ions.
表格
| Stress Source | Failure Prone Area | Elimination Method |
|---|---|---|
| Cold bending forming | Outer arc of bent section | Limit bending radius + stress relief annealing |
| Welding processing | Weld and heat-affected zone | Double-sided argon protection + post-weld heat treatment |
| Rigid installation | Flange connection part | Flexible gasket + reserved expansion allowance |
| Thermal frequent start-stop | Whole tube body | Reduce power density and stabilize temperature fluctuation |
Conclusion
TA2 titanium heating tubes do not have absolute immunity to stress corrosion cracking in high-temperature and high-salinity environments. Cutting off tensile stress from forming, welding and assembly links, together with temperature and medium management, can effectively avoid sudden brittle cracking failure and give full play to titanium's long-life anti-corrosion advantage in chloride-rich working conditions.
